The present invention relates to atomic force microscopy. In one aspect, it relates to atomic force microscopy scanning methods that may be useful for evaluating deep features of semiconductor products.
Atomic force microscopy (AFM) probes are often used to evaluate and measure features on a semiconductor product as the semiconductor product is being developed or fabricated into an integrated circuit device, for example. Conventional AFM probes typically include a silicon cantilever beam with a silicon tip (xe2x80x9cAFM tipxe2x80x9d) extending perpendicular to or at a slight angle (e.g., 10 degrees) relative to the cantilever beam. The tip is often formed into a long and thin rod. The silicon tip is often etched to form a sharp apex small enough to fit into a deep feature. There are several high aspect ratio tips on the market made for imaging and measuring deep narrow features. Some high aspect ratio tips are made using a focused ion beam to machine the silicon tip into a long thin rod with an aspect ratio between 7:1 and 10:1. Hence, an AFM tip with a 10:1 aspect ratio (i.e., length:diameter) may be able to reach 1000 nm into a 100 nm diameter trench. Other high aspect ratio tips may be formed using electron beam deposition (e.g., EBD tips) or may be carbon nanotubes with a diameter between 10 nm and 80 nm, for example.
The AFM tip is sometimes scanned across a sample surface to create an image of the detected surface features. Special tips are often made having a high aspect ratio, which allows them to be lowered into small diameter features without touching the sidewalls.
As technology progresses, the features of integrated circuits typically become smaller, and in some cases, deeper. Thus, the demands on the size and precision of movement of AFM probe tips tends to increase as well. Problems that often occur in use of such AFM probes include rapid wear of the AFM tip and breaking the AFM tip. Such wear often occurs when the AFM tip hits a sidewall, sticks in a steep surface, or is dragged across a sidewall or surface while maneuvering the AFM tip. AFM probe cost and tool downtime makes it desirable to extend the usable life of an AFM tip. Hence, there is a need for a way to reduce AFM probe tip wear and extend the usable life of an AFM probe. The production of semiconductors often requires measurements of small-diameter, deep features. Hence, there is also a need to reduce AFM tip wear, and AFM tip sticking, while increasing the AFM depth measurement.
The problems and needs outlined above are addressed by certain aspects of the present invention. In accordance with one aspect of the present invention, a method of scanning a deep feature extending into a surface using an atomic force microscopy (AFM) tip is provided. The method includes the following steps. First, the deep feature is located and mapped with a surface survey scan. Second, data from the surface survey scan is analyzed to identify an initial optimum location for probing into the deep feature with the AFM tip. Third, the AFM tip is moved to the initial optimum location approximately at surface level. Fourth, a first procedure is repeated until the AFM tip reaches a bottom of the deep feature. The first procedure includes the following steps: (i) lowering the AFM tip in a first direction parallel with a first axis extending into the deep feature by a first distance increment; (ii) measuring atomic force interactions exerted on the AFM tip to determine whether the bottom of the deep feature has been reached; (iii) moving the AFM tip in a geometric pattern and within a current plane at a current location along the first axis, the current plane being substantially perpendicular to the first axis; (iv) measuring atomic force interactions exerted on the AFM tip at various locations in the geometric pattern to determine a new optimum location in the geometric pattern where the atomic force interactions are at a minimum; and (v) moving the AFM tip within the current plane to the new optimum location. The method may further include the step of calculating a depth of the deep feature after the AFM tip reaches the bottom of the deep feature. The deep feature may be a trench formed in a semiconductor wafer while fabricating a capacitor for an integrated circuit, for example. Hence, the method may be used to test an integrated circuit device during production.
In accordance with another aspect of the present invention, a method of scanning a deep feature extending into a surface using an AFM tip is provided. This method includes the following steps. First, the deep feature is located and mapped with a surface survey scan. Second, data from the surface survey scan is analyzed to identify an initial optimum location for probing into the deep feature with the AFM tip. Third, the AFM tip is moved to the initial optimum location approximately at surface level. Fourth, a first procedure is repeated until the AFM tip reaches a bottom of the deep feature. The first procedure includes the following steps: (a) lowering the AFM tip in a first direction parallel with a first axis extending into the deep feature by a first distance increment; (b) measuring atomic force interactions exerted on the AFM tip to determine whether the bottom of the deep feature has been reached; (c) moving the AFM tip in a geometric pattern and within a current plane at a current location along the first axis the current plane being substantially perpendicular to the first axis; (d) measuring atomic force interactions exerted on the AFM tip at various locations in the geometric pattern to determine a new optimum location in the geometric pattern where the atomic force interactions are at a minimum; and (e) moving the AFM tip within the current plane to the new optimum location. Fifth, a depth of the deep feature is calculated after the AFM tip reaches the bottom of the deep feature. A second procedure used for exiting the AFM tip from the deep feature and moving across the surface includes the following steps: (a) First raise the tip so it is not in-contact with the surface. (b) Move the AFM tip in the second direction parallel to the surface, which is towards the next location set for measuring the surface height. (c) Repeating the second movement increment until the AFM tip reaches the next location or nears an object on the surface. (d) If an object is encountered then move one increment in reverse to the second movement. (e) Repeat step second procedure a-d until the next location is reached. This method also may be used to test an integrated circuit device during production.
In accordance with yet another aspect of the present invention, a method of fabricating an integrated circuit device is provided. The method includes the following steps. First, a trench is formed in a semiconductor wafer. The trench extends into a surface of the wafer. Second, a depth of the trench is measured using a method of scanning the trench with an atomic force microscopy (AFM) tip. The method of scanning includes: (i) locating and mapping the trench with a surface survey scan; (ii) analyzing data from the surface survey scan to identify an initial optimum location for probing into the trench with the AFM tip; (iii) moving the AFM tip to the initial optimum location approximately at surface level; and (iv) repeating a first procedure until the AFM tip reaches a bottom of the trench, the first procedure including: (a) lowering the AFM tip in a first direction into the trench by a first distance increment, (b) measuring atomic force interactions exerted on the AFM tip to determine whether the bottom of the trench has been reached, (c) moving the AFM tip in a geometric pattern and within a current plane at a current location along the depth His axis the current plane being that are perpendicular to the first axis, (d) measuring atomic force interactions exerted on the AFM tip at various locations in the geometric pattern to determine a new optimum location in the geometric pattern where the atomic force interactions are at a minimum, and (e) move the AFM tip within the current plane to the new optimum location. The trench may be partially filled with polysilicon before measuring the depth of the trench. This method may further includes the steps of: plotting a top view of an approximate trench outline based on the analysis of the surface survey scan, and/or plotting an approximate center location of the trench outline based on the analysis of the surface survey scan. Also, this method may further include: calculating a depth of the trench after the AFM tip reaches the bottom of the trench and displaying the depth proximate to the plot of the trench outline. Hence, an integrated circuit device may be tested during production using this method.